1. Field of the Invention
The invention relates to a method of separating a foreground picture signal FG from a mixed picture signal MG which has a key color KC as well as transition colors to this key color in key regions, the foreground picture signal being removed from the key color and the transition colors in accordance with the equation EQU FG=MG-k*KC
in which k is a control signal which has a value of about 1 in the region of the key color KC and a value of about 0 at the transition boundaries, the transition boundaries in the C.sub.b /C.sub.r color plane being constituted by two straight lines through the origin of the C.sub.b /C.sub.r color plane which, relative to a vector D.sub.KC of the key color extending from the origin of the C.sub.b /C.sub.r color plane to the color location KC of the key color, are arranged at angles .omega..sub.o and .omega..sub.u, respectively.
2. Description of the Related Art
Such a method is known from GB-A-2 044 036. The method can be used for removing the key color occurring in the key regions of the mixed picture signal, as well as transition colors occurring between the foreground picture signal and the key color, and possibly replacing these regions by a background picture signal. A particular problem in this respect is to recognize the transition colors as such and to suppress them accordingly. The limit up to which parts of the key color or transition colors are removed from the mixed picture signal is constituted by transition boundaries which are formed by two straight lines through the origin of the C.sub.b /C.sub.r color plane in the C.sub.b /C.sub.r color plane. All the color locations between these straight lines through the origin of the C.sub.b /C.sup.r color plane and the color location KC of the key color are considered as key colors or as transition colors. Exclusively in the region between the key color and the transition boundaries, a pan of the key color is removed from the mixed picture signal for generating the foreground picture signal. The transition boundaries are the straight lines through the origin of the C.sub.b /C.sub.r color plane. Relative to the vector D.sub.KC, these lines are arranged at angles .omega..sub.o and .omega..sub.u of the C.sub.b /C.sub.r color plane. The vector D.sub.KC extends from the origin of the C.sub.b /C.sub.r color plane to the color location KC of the key color. The angles .omega..sub.o and .omega..sub.u are the angles between the vector D.sub.KC and the two straight lines through the origin of the C.sub.b /C.sub.r color plane.